优雅关闭与清理
清单 20-20 中的代码正在通过使用线程池异步响应请求，正如我们预期的那样。我们收到一些关于 workers、id 和 thread 字段的警告，这些字段我们没有以直接的方式使用，这提醒我们我们没有清理任何东西。当我们使用不太优雅的 ctrl-c 方法停止主线程时，所有其他线程也会立即停止，即使它们正在处理请求。
接下来，我们将实现 Drop trait 以在池中每个线程上调用 join，以便它们可以在关闭之前完成正在处理的请求。然后，我们将实现一种方法来告诉线程它们应该停止接受新请求并关闭。为了看到这段代码的实际效果，我们将修改我们的服务器，使其在优雅地关闭其线程池之前仅接受两个请求。
在 ThreadPool 上实现 Drop Trait
让我们从在线程池上实现 Drop 开始。当池被 drop 时，我们的线程应该全部 join 以确保它们完成工作。清单 20-22 展示了 Drop 实现的首次尝试；这段代码还不能完全工作。
文件名: src/lib.rs
use std::{
sync::{mpsc, Arc, Mutex},
thread,
};
pub struct ThreadPool {
workers: Vec<Worker>,
sender: mpsc::Sender<Job>,
}
type Job = Box<dyn FnOnce() + Send + 'static>;
impl ThreadPool {
/// Create a new ThreadPool.
///
/// The size is the number of threads in the pool.
///
/// # Panics
///
/// The `new` function will panic if the size is zero.
pub fn new(size: usize) -> ThreadPool {
assert!(size > 0);
let (sender, receiver) = mpsc::channel();
let receiver = Arc::new(Mutex::new(receiver));
let mut workers = Vec::with_capacity(size);
for id in 0..size {
workers.push(Worker::new(id, Arc::clone(&receiver)));
}
ThreadPool { workers, sender }
}
pub fn execute<F>(&self, f: F)
where
F: FnOnce() + Send + 'static,
{
let job = Box::new(f);
self.sender.send(job).unwrap();
}
}
impl Drop for ThreadPool {
fn drop(&mut self) {
for worker in &mut self.workers {
println!("Shutting down worker {}", worker.id);
worker.thread.join().unwrap();
}
}
}
struct Worker {
id: usize,
thread: thread::JoinHandle<()>,
}
impl Worker {
fn new(id: usize, receiver: Arc<Mutex<mpsc::Receiver<Job>>>) -> Worker {
let thread = thread::spawn(move || loop {
let job = receiver.lock().unwrap().recv().unwrap();
println!("Worker {id} got a job; executing.");
job();
});
Worker { id, thread }
}
}清单 20-22：当线程池超出作用域时 join 每个线程
首先，我们循环遍历线程池中的每个 workers。我们为此使用 &mut，因为 self 是一个可变引用，并且我们也需要能够改变 worker。对于每个 worker，我们打印一条消息，说明这个特定的 worker 正在关闭，然后我们对该 worker 的线程调用 join。如果调用 join 失败，我们使用 unwrap 使 Rust panic 并进入不优雅的关闭。
这是我们编译这段代码时得到的错误
$ cargo check
Checking hello v0.1.0 (file:///projects/hello)
error[E0507]: cannot move out of `worker.thread` which is behind a mutable reference
--> src/lib.rs:52:13
|
52 | worker.thread.join().unwrap();
| ^^^^^^^^^^^^^ ------ `worker.thread` moved due to this method call
| |
| move occurs because `worker.thread` has type `JoinHandle<()>`, which does not implement the `Copy` trait
|
note: `JoinHandle::<T>::join` takes ownership of the receiver `self`, which moves `worker.thread`
--> /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/std/src/thread/mod.rs:1778:17
For more information about this error, try `rustc --explain E0507`.
error: could not compile `hello` (lib) due to 1 previous error
该错误告诉我们无法调用 join，因为我们只拥有每个 worker 的可变借用，而 join 取得其参数的所有权。为了解决这个问题，我们需要将线程从拥有 thread 的 Worker 实例中移出，以便 join 可以消费该线程。我们在清单 17-15 中做过这件事：如果 Worker 持有一个 Option<thread::JoinHandle<()>>，我们可以对 Option 调用 take 方法，将值从 Some 变体中移出，并在其位置留下 None 变体。换句话说，正在运行的 Worker 将在 thread 中拥有一个 Some 变体，当我们想要清理 Worker 时，我们将用 None 替换 Some，以便 Worker 没有线程可以运行。
所以我们知道我们想要像这样更新 Worker 的定义
文件名: src/lib.rs
use std::{
sync::{mpsc, Arc, Mutex},
thread,
};
pub struct ThreadPool {
workers: Vec<Worker>,
sender: mpsc::Sender<Job>,
}
type Job = Box<dyn FnOnce() + Send + 'static>;
impl ThreadPool {
/// Create a new ThreadPool.
///
/// The size is the number of threads in the pool.
///
/// # Panics
///
/// The `new` function will panic if the size is zero.
pub fn new(size: usize) -> ThreadPool {
assert!(size > 0);
let (sender, receiver) = mpsc::channel();
let receiver = Arc::new(Mutex::new(receiver));
let mut workers = Vec::with_capacity(size);
for id in 0..size {
workers.push(Worker::new(id, Arc::clone(&receiver)));
}
ThreadPool { workers, sender }
}
pub fn execute<F>(&self, f: F)
where
F: FnOnce() + Send + 'static,
{
let job = Box::new(f);
self.sender.send(job).unwrap();
}
}
impl Drop for ThreadPool {
fn drop(&mut self) {
for worker in &mut self.workers {
println!("Shutting down worker {}", worker.id);
worker.thread.join().unwrap();
}
}
}
struct Worker {
id: usize,
thread: Option<thread::JoinHandle<()>>,
}
impl Worker {
fn new(id: usize, receiver: Arc<Mutex<mpsc::Receiver<Job>>>) -> Worker {
let thread = thread::spawn(move || loop {
let job = receiver.lock().unwrap().recv().unwrap();
println!("Worker {id} got a job; executing.");
job();
});
Worker { id, thread }
}
}现在让我们依靠编译器来找到其他需要更改的地方。检查这段代码，我们得到两个错误
$ cargo check
Checking hello v0.1.0 (file:///projects/hello)
error[E0599]: no method named `join` found for enum `Option` in the current scope
--> src/lib.rs:52:27
|
52 | worker.thread.join().unwrap();
| ^^^^ method not found in `Option<JoinHandle<()>>`
|
note: the method `join` exists on the type `JoinHandle<()>`
--> /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/std/src/thread/mod.rs:1778:5
help: consider using `Option::expect` to unwrap the `JoinHandle<()>` value, panicking if the value is an `Option::None`
|
52 | worker.thread.expect("REASON").join().unwrap();
| +++++++++++++++++
error[E0308]: mismatched types
--> src/lib.rs:72:22
|
72 | Worker { id, thread }
| ^^^^^^ expected `Option<JoinHandle<()>>`, found `JoinHandle<_>`
|
= note: expected enum `Option<JoinHandle<()>>`
found struct `JoinHandle<_>`
help: try wrapping the expression in `Some`
|
72 | Worker { id, thread: Some(thread) }
| +++++++++++++ +
Some errors have detailed explanations: E0308, E0599.
For more information about an error, try `rustc --explain E0308`.
error: could not compile `hello` (lib) due to 2 previous errors
让我们解决第二个错误，它指向 Worker::new 末尾的代码；当创建一个新的 Worker 时，我们需要将 thread 值包装在 Some 中。进行以下更改以修复此错误
文件名: src/lib.rs
use std::{
sync::{mpsc, Arc, Mutex},
thread,
};
pub struct ThreadPool {
workers: Vec<Worker>,
sender: mpsc::Sender<Job>,
}
type Job = Box<dyn FnOnce() + Send + 'static>;
impl ThreadPool {
/// Create a new ThreadPool.
///
/// The size is the number of threads in the pool.
///
/// # Panics
///
/// The `new` function will panic if the size is zero.
pub fn new(size: usize) -> ThreadPool {
assert!(size > 0);
let (sender, receiver) = mpsc::channel();
let receiver = Arc::new(Mutex::new(receiver));
let mut workers = Vec::with_capacity(size);
for id in 0..size {
workers.push(Worker::new(id, Arc::clone(&receiver)));
}
ThreadPool { workers, sender }
}
pub fn execute<F>(&self, f: F)
where
F: FnOnce() + Send + 'static,
{
let job = Box::new(f);
self.sender.send(job).unwrap();
}
}
impl Drop for ThreadPool {
fn drop(&mut self) {
for worker in &mut self.workers {
println!("Shutting down worker {}", worker.id);
worker.thread.join().unwrap();
}
}
}
struct Worker {
id: usize,
thread: Option<thread::JoinHandle<()>>,
}
impl Worker {
fn new(id: usize, receiver: Arc<Mutex<mpsc::Receiver<Job>>>) -> Worker {
// --snip--
let thread = thread::spawn(move || loop {
let job = receiver.lock().unwrap().recv().unwrap();
println!("Worker {id} got a job; executing.");
job();
});
Worker {
id,
thread: Some(thread),
}
}
}第一个错误在我们的 Drop 实现中。我们之前提到过，我们打算对 Option 值调用 take，以将 thread 从 worker 中移出。以下更改将实现这一点
文件名: src/lib.rs
use std::{
sync::{mpsc, Arc, Mutex},
thread,
};
pub struct ThreadPool {
workers: Vec<Worker>,
sender: mpsc::Sender<Job>,
}
type Job = Box<dyn FnOnce() + Send + 'static>;
impl ThreadPool {
/// Create a new ThreadPool.
///
/// The size is the number of threads in the pool.
///
/// # Panics
///
/// The `new` function will panic if the size is zero.
pub fn new(size: usize) -> ThreadPool {
assert!(size > 0);
let (sender, receiver) = mpsc::channel();
let receiver = Arc::new(Mutex::new(receiver));
let mut workers = Vec::with_capacity(size);
for id in 0..size {
workers.push(Worker::new(id, Arc::clone(&receiver)));
}
ThreadPool { workers, sender }
}
pub fn execute<F>(&self, f: F)
where
F: FnOnce() + Send + 'static,
{
let job = Box::new(f);
self.sender.send(job).unwrap();
}
}
impl Drop for ThreadPool {
fn drop(&mut self) {
for worker in &mut self.workers {
println!("Shutting down worker {}", worker.id);
if let Some(thread) = worker.thread.take() {
thread.join().unwrap();
}
}
}
}
struct Worker {
id: usize,
thread: Option<thread::JoinHandle<()>>,
}
impl Worker {
fn new(id: usize, receiver: Arc<Mutex<mpsc::Receiver<Job>>>) -> Worker {
let thread = thread::spawn(move || loop {
let job = receiver.lock().unwrap().recv().unwrap();
println!("Worker {id} got a job; executing.");
job();
});
Worker {
id,
thread: Some(thread),
}
}
}正如第 18 章讨论的那样，Option 上的 take 方法取出 Some 变体并在其位置留下 None。我们使用 if let 来解构 Some 并获取线程；然后我们对线程调用 join。如果 worker 的线程已经是 None，我们知道该 worker 已经清理了它的线程，所以在这种情况下什么也不会发生。
向线程发送信号以停止监听作业
通过我们所做的所有更改，我们的代码编译时没有任何警告。然而，坏消息是这段代码还不能按照我们想要的方式运行。关键在于 Worker 实例的线程运行的闭包中的逻辑：目前，我们调用 join，但这不会关闭线程，因为它们会永远 loop 寻找作业。如果我们尝试使用我们当前的 drop 实现来 drop 我们的 ThreadPool，主线程将永远阻塞，等待第一个线程完成。
为了解决这个问题，我们需要更改 ThreadPool drop 实现，然后更改 Worker 循环。
首先，我们将更改 ThreadPool drop 实现，以在等待线程完成之前显式 drop sender。清单 20-23 显示了对 ThreadPool 的更改，以显式 drop sender。我们使用与线程相同的 Option 和 take 技术，以便能够将 sender 从 ThreadPool 中移出
文件名: src/lib.rs
use std::{
sync::{mpsc, Arc, Mutex},
thread,
};
pub struct ThreadPool {
workers: Vec<Worker>,
sender: Option<mpsc::Sender<Job>>,
}
// --snip--
type Job = Box<dyn FnOnce() + Send + 'static>;
impl ThreadPool {
/// Create a new ThreadPool.
///
/// The size is the number of threads in the pool.
///
/// # Panics
///
/// The `new` function will panic if the size is zero.
pub fn new(size: usize) -> ThreadPool {
// --snip--
assert!(size > 0);
let (sender, receiver) = mpsc::channel();
let receiver = Arc::new(Mutex::new(receiver));
let mut workers = Vec::with_capacity(size);
for id in 0..size {
workers.push(Worker::new(id, Arc::clone(&receiver)));
}
ThreadPool {
workers,
sender: Some(sender),
}
}
pub fn execute<F>(&self, f: F)
where
F: FnOnce() + Send + 'static,
{
let job = Box::new(f);
self.sender.as_ref().unwrap().send(job).unwrap();
}
}
impl Drop for ThreadPool {
fn drop(&mut self) {
drop(self.sender.take());
for worker in &mut self.workers {
println!("Shutting down worker {}", worker.id);
if let Some(thread) = worker.thread.take() {
thread.join().unwrap();
}
}
}
}
struct Worker {
id: usize,
thread: Option<thread::JoinHandle<()>>,
}
impl Worker {
fn new(id: usize, receiver: Arc<Mutex<mpsc::Receiver<Job>>>) -> Worker {
let thread = thread::spawn(move || loop {
let job = receiver.lock().unwrap().recv().unwrap();
println!("Worker {id} got a job; executing.");
job();
});
Worker {
id,
thread: Some(thread),
}
}
}清单 20-23：在 join worker 线程之前显式 drop sender
Dropping sender 会关闭通道，这表明不会再发送更多消息。当这种情况发生时，worker 在无限循环中所做的所有 recv 调用都将返回错误。在清单 20-24 中，我们更改了 Worker 循环以在这种情况下优雅地退出循环，这意味着当 ThreadPool drop 实现对它们调用 join 时，线程将完成。
文件名: src/lib.rs
use std::{
sync::{mpsc, Arc, Mutex},
thread,
};
pub struct ThreadPool {
workers: Vec<Worker>,
sender: Option<mpsc::Sender<Job>>,
}
type Job = Box<dyn FnOnce() + Send + 'static>;
impl ThreadPool {
/// Create a new ThreadPool.
///
/// The size is the number of threads in the pool.
///
/// # Panics
///
/// The `new` function will panic if the size is zero.
pub fn new(size: usize) -> ThreadPool {
assert!(size > 0);
let (sender, receiver) = mpsc::channel();
let receiver = Arc::new(Mutex::new(receiver));
let mut workers = Vec::with_capacity(size);
for id in 0..size {
workers.push(Worker::new(id, Arc::clone(&receiver)));
}
ThreadPool {
workers,
sender: Some(sender),
}
}
pub fn execute<F>(&self, f: F)
where
F: FnOnce() + Send + 'static,
{
let job = Box::new(f);
self.sender.as_ref().unwrap().send(job).unwrap();
}
}
impl Drop for ThreadPool {
fn drop(&mut self) {
drop(self.sender.take());
for worker in &mut self.workers {
println!("Shutting down worker {}", worker.id);
if let Some(thread) = worker.thread.take() {
thread.join().unwrap();
}
}
}
}
struct Worker {
id: usize,
thread: Option<thread::JoinHandle<()>>,
}
impl Worker {
fn new(id: usize, receiver: Arc<Mutex<mpsc::Receiver<Job>>>) -> Worker {
let thread = thread::spawn(move || loop {
let message = receiver.lock().unwrap().recv();
match message {
Ok(job) => {
println!("Worker {id} got a job; executing.");
job();
}
Err(_) => {
println!("Worker {id} disconnected; shutting down.");
break;
}
}
});
Worker {
id,
thread: Some(thread),
}
}
}清单 20-24：当 recv 返回错误时显式跳出循环
为了看到这段代码的实际效果，让我们修改 main 以在优雅地关闭服务器之前仅接受两个请求，如清单 20-25 所示。
文件名: src/main.rs
use hello::ThreadPool;
use std::{
fs,
io::{prelude::*, BufReader},
net::{TcpListener, TcpStream},
thread,
time::Duration,
};
fn main() {
let listener = TcpListener::bind("127.0.0.1:7878").unwrap();
let pool = ThreadPool::new(4);
for stream in listener.incoming().take(2) {
let stream = stream.unwrap();
pool.execute(|| {
handle_connection(stream);
});
}
println!("Shutting down.");
}
fn handle_connection(mut stream: TcpStream) {
let buf_reader = BufReader::new(&stream);
let request_line = buf_reader.lines().next().unwrap().unwrap();
let (status_line, filename) = match &request_line[..] {
"GET / HTTP/1.1" => ("HTTP/1.1 200 OK", "hello.html"),
"GET /sleep HTTP/1.1" => {
thread::sleep(Duration::from_secs(5));
("HTTP/1.1 200 OK", "hello.html")
}
_ => ("HTTP/1.1 404 NOT FOUND", "404.html"),
};
let contents = fs::read_to_string(filename).unwrap();
let length = contents.len();
let response =
format!("{status_line}\r\nContent-Length: {length}\r\n\r\n{contents}");
stream.write_all(response.as_bytes()).unwrap();
}清单 20-25：通过退出循环在服务两个请求后关闭服务器
你不会希望一个真实的 Web 服务器在仅服务两个请求后就关闭。这段代码只是演示了优雅关闭和清理正在正常工作。
take 方法在 Iterator trait 中定义，并将迭代限制为最多前两个条目。ThreadPool 将在 main 的末尾超出作用域，并且 drop 实现将运行。
使用 cargo run 启动服务器，并发出三个请求。第三个请求应该报错，并且在你的终端中，你应该看到类似于这样的输出
$ cargo run
Compiling hello v0.1.0 (file:///projects/hello)
Finished dev [unoptimized + debuginfo] target(s) in 1.0s
Running `target/debug/hello`
Worker 0 got a job; executing.
Shutting down.
Shutting down worker 0
Worker 3 got a job; executing.
Worker 1 disconnected; shutting down.
Worker 2 disconnected; shutting down.
Worker 3 disconnected; shutting down.
Worker 0 disconnected; shutting down.
Shutting down worker 1
Shutting down worker 2
Shutting down worker 3
你可能会看到 worker 和消息打印的不同顺序。我们可以从消息中看到这段代码是如何工作的：worker 0 和 3 获取了前两个请求。服务器在第二个连接后停止接受连接，并且 ThreadPool 上的 Drop 实现甚至在 worker 3 开始其工作之前就开始执行。Dropping sender 断开了所有 worker 的连接，并告诉它们关闭。worker 在断开连接时各自打印一条消息，然后线程池调用 join 以等待每个 worker 线程完成。
请注意此特定执行的一个有趣的方面：ThreadPool dropped 了 sender，并且在任何 worker 收到错误之前，我们尝试 join worker 0。Worker 0 尚未从 recv 收到错误，因此主线程阻塞等待 worker 0 完成。与此同时，worker 3 收到一个作业，然后所有线程都收到一个错误。当 worker 0 完成时，主线程等待其余 worker 完成。那时，它们都退出了循环并停止了。
恭喜！我们现在已经完成了我们的项目；我们有一个基本的 Web 服务器，它使用线程池来异步响应。我们能够对服务器执行优雅关闭，这会清理池中的所有线程。
这是完整的代码供参考
文件名: src/main.rs
use hello::ThreadPool;
use std::{
fs,
io::{prelude::*, BufReader},
net::{TcpListener, TcpStream},
thread,
time::Duration,
};
fn main() {
let listener = TcpListener::bind("127.0.0.1:7878").unwrap();
let pool = ThreadPool::new(4);
for stream in listener.incoming().take(2) {
let stream = stream.unwrap();
pool.execute(|| {
handle_connection(stream);
});
}
println!("Shutting down.");
}
fn handle_connection(mut stream: TcpStream) {
let buf_reader = BufReader::new(&stream);
let request_line = buf_reader.lines().next().unwrap().unwrap();
let (status_line, filename) = match &request_line[..] {
"GET / HTTP/1.1" => ("HTTP/1.1 200 OK", "hello.html"),
"GET /sleep HTTP/1.1" => {
thread::sleep(Duration::from_secs(5));
("HTTP/1.1 200 OK", "hello.html")
}
_ => ("HTTP/1.1 404 NOT FOUND", "404.html"),
};
let contents = fs::read_to_string(filename).unwrap();
let length = contents.len();
let response =
format!("{status_line}\r\nContent-Length: {length}\r\n\r\n{contents}");
stream.write_all(response.as_bytes()).unwrap();
}文件名: src/lib.rs
use std::{
sync::{mpsc, Arc, Mutex},
thread,
};
pub struct ThreadPool {
workers: Vec<Worker>,
sender: Option<mpsc::Sender<Job>>,
}
type Job = Box<dyn FnOnce() + Send + 'static>;
impl ThreadPool {
/// Create a new ThreadPool.
///
/// The size is the number of threads in the pool.
///
/// # Panics
///
/// The `new` function will panic if the size is zero.
pub fn new(size: usize) -> ThreadPool {
assert!(size > 0);
let (sender, receiver) = mpsc::channel();
let receiver = Arc::new(Mutex::new(receiver));
let mut workers = Vec::with_capacity(size);
for id in 0..size {
workers.push(Worker::new(id, Arc::clone(&receiver)));
}
ThreadPool {
workers,
sender: Some(sender),
}
}
pub fn execute<F>(&self, f: F)
where
F: FnOnce() + Send + 'static,
{
let job = Box::new(f);
self.sender.as_ref().unwrap().send(job).unwrap();
}
}
impl Drop for ThreadPool {
fn drop(&mut self) {
drop(self.sender.take());
for worker in &mut self.workers {
println!("Shutting down worker {}", worker.id);
if let Some(thread) = worker.thread.take() {
thread.join().unwrap();
}
}
}
}
struct Worker {
id: usize,
thread: Option<thread::JoinHandle<()>>,
}
impl Worker {
fn new(id: usize, receiver: Arc<Mutex<mpsc::Receiver<Job>>>) -> Worker {
let thread = thread::spawn(move || loop {
let message = receiver.lock().unwrap().recv();
match message {
Ok(job) => {
println!("Worker {id} got a job; executing.");
job();
}
Err(_) => {
println!("Worker {id} disconnected; shutting down.");
break;
}
}
});
Worker {
id,
thread: Some(thread),
}
}
}我们可以在这里做更多的事情！如果你想继续增强这个项目，这里有一些想法
- 向
ThreadPool及其公共方法添加更多文档。 - 添加库功能的测试。
- 将对
unwrap的调用更改为更健壮的错误处理。 - 使用
ThreadPool执行一些除服务 Web 请求之外的任务。 - 在 crates.io 上找到一个线程池 crate，并使用该 crate 实现一个类似的 Web 服务器。然后将其 API 和健壮性与我们实现的线程池进行比较。
总结
做得好！你已经读到本书的结尾了！我们要感谢你加入我们的 Rust 之旅。你现在已经准备好实现你自己的 Rust 项目并帮助他人的项目。请记住，有一个热情的 Rustacean 社区，他们很乐意帮助你解决你在 Rust 之旅中遇到的任何挑战。